The present invention relates to light emitting diodes and in particular relates to increasing the external quantum efficiency of light emitting diodes that include at least portions of the substrates on which the light emitting epitaxial layer's are grown.
Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied voltage into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost.
A number of terms are used herein that are common and well-understood in the industry. In such industry use, however, these terms are sometimes informally blended in their meaning. Accordingly, these terms will be used as precisely as possible herein, but in every case their meaning will be clear in context.
Accordingly, the term “diode” or “chip” typically refers to the structure that minimally includes two semiconductor portions of opposite conductivity types (p and n) along with some form of ohmic contacts to permit current to be applied across the resulting p-n junction.
The term “lamp” is used to designate a light emitting diode that is matched with an appropriate electrical contact and potentially a lens to form a discrete device that can be added to or included in electrical circuits or lighting fixtures or both.
As used herein, the term “package” typically refers to the placement of the semiconductor chip on an appropriate physical and electrical structure (sometimes as simple as a small piece of metal through which the electrical current is applied) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output.
Appropriate references about the structure and operation of light emitting diodes and diode lamps include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition (1981) and Schubert, LIGHT-EMITTING DIODES, Cambridge University Press (2003).
The color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy (red and yellow) portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue, violet and ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. As the availability of blue-emitting LEDs has greatly increased, the incorporation of yellow-emitting phosphors that down-convert the blue photons has likewise increased. The combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as lighting (frequently backlighting) for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color pixels. Such color pixels are often formed by a combination of liquid crystals, color filters and polarizers, and the entire unit including the backlighting is generally referred to as a liquid crystal display. (“LCD”).
As the use of light emitting diodes has commercially increased and as the understanding of the basic characteristics of diodes that produce white light has matured, the advances of interest in the technology tend to be those that increase the total amount of light that is produced by a given diode structure, all other factors being equal.
In this regard, the number of individual photons produced by a diode in any given amount of time depends upon the number of recombination events occurring in the diode, with the number of photons generally being less than the number of recombination events (i.e., not every event produces a photon). In turn, the number of recombination events depends upon the amount of current flowing through the diode. Once again the number of recombination events will typically be less than the number of carriers injected across the junction. Thus, these optoelectronic properties can reduce the external output of the diode.
Additionally, when photons are produced, they must also actually leave the diode and the lamp to be perceived by an observer. Although many photons will leave the lamp without difficulty, a number of well-understood effects prevent some fraction of the photons from leaving. These effects arise from the difference in refractive index of the various materials within the diode, and thus reduce the external output of an LED lamp (i.e., its efficiency). These include internal reflection of a photon until it attenuated and emitted or absorbed (i.e., Snell's Law and Fresnel Loss) rather than emitted. The difference in the index of refraction between the materials in the diode can also change the direction of an emitted photon (Snell's Law) towards an object that subsequently attenuates or absorbs it. The same results can occur for yellow photons that are emitted by the phosphor in a phosphor-containing LED lamp. In an LED lamp such “objects” can include the substrate, parts of the packaging, and the metal contact layers. Indeed, the same quantum mechanical characteristics that permit semiconductor materials to emit photons will also cause them to absorb photons. Thus, even the light emitting epitaxial layers in an LED can absorb emitted photons and reduce the overall external efficiency of the diode.
Many semiconductor devices, including many light emitting diodes, consist in basic form of a semiconductor substrate and epitaxial layers of semiconductor materials on the substrate. The epitaxial layers often (although not necessarily exclusively) form the active portions of the device. They are generally favored for this purpose because they are grown in a manner (frequently chemical vapor deposition) that increases both their chemical purity and produces a highly ordered crystal structure. Additionally, chemical vapor deposition provides an excellent technique for precisely doping an epitaxial layer. In turn, the appropriate purity, crystal structure and doping are typically desired or necessary for successful operation of the semiconductor device.
The chemical vapor deposition (CVD) and related techniques used to fabricate epitaxial layers are, however, generally more time-consuming than other crystal growth techniques such as sublimation or growth from a melt (sometimes referred to as bulk growth). As a result, these more rapid (comparatively) methods are often used to produce an appropriate crystal when the intended structure is other than an epitaxial layer.
Thus, by combining a bulk-growth substrate with epitaxial layers, an overall structure can be produced with a reasonable combination of crystal structure, compositional purity, doping, and efficient fabrication.
Nevertheless, for several crystal growth-related reasons, bulk (i.e., reasonably large size) single crystals of Group III nitrides are, for practical purposes, unavailable. Accordingly, Group III nitride LEDs are typically formed on other bulk substrate materials, most commonly sapphire (Al2O3) and silicon carbide (SiC). Sapphire is relatively inexpensive, widely available, and highly transparent. Alternatively, sapphire is a poor thermal conductor and therefore less suitable for certain high-power applications. Additionally, in some devices, electrically conductive substrates are preferred and sapphire is insulating rather than conductive. Sapphire also carries a lattice mismatch with (for example) gallium nitride of about 16%.
Silicon carbide has a better thermal conductivity than sapphire and a better lattice match with Group IIII nitrides; i.e., a mismatch of about 3.5% with gallium nitride and only about 1% with aluminum nitride. Silicon carbide can be conductively doped, but is also much more expensive than sapphire.
Thus, depending upon the desired application, both sapphire and silicon carbide can provide an appropriate substrate for Group III nitride epitaxial layers in light emitting diodes.
In almost all cases, using a substrate material that is different from the epitaxial layer material creates an additional set of problems due to different thermal coefficients of expansion (TCEs) and different crystal lattice parameters. As a result, when Group III nitride epitaxial layers are grown on a different substrate, some crystal mismatch will occur, and the resulting epitaxial layer is referred to as being “strained” either in tension or compression by these mismatches. Such mismatches, and the strain they produce, carry with them the potential for crystal defects which in turn affect the optoelectronic characteristics of the crystals and the junctions, and thus correspondingly tend to degrade or even prevent the performance of the photonic device.
The presence of multiple layers of different materials (substrate, epilayers, metal contacts) in light emitting diodes raises additional issues. In particular, light emitted from the active portion must typically pass through or across one or more of such layers before exiting the diode. Additionally, when the diode is packaged as a lamp, the light leaving the diode must travel into, through, and out of the lens material. In each of these circumstances, Snell's law dictates that the photons will be refracted as they pass from one material to the next. The amount that the photons are refracted will depend upon the difference between the refractive indexes of the two materials and the angle of incidence at which the light strikes the interface. Additionally, in almost all circumstances, some of the photons (even if a small percentage) will always be reflected at the interface between the two materials. This is referred to as Fresnel reflection or Fresnel loss.
In a diode or a diode lamp, although some reflected light will still escape the diode at some other location, a certain percentage will be totally internally reflected, never escape the diode or the lamp, and will thus functionally reduce the external quantum efficiency of the diode and of any lamp that includes the diode. Although the individual reduction in the percentage of photons escaping may appear to be relatively small, the cumulative effect can be significant and diodes that are otherwise very similar can have distinctly different performance efficiencies resulting from even these small percentage losses.
Accordingly, increasing the external efficiency of white light emitting diodes remains a continuing goal.